1) Field of the Invention
The present invention relates to charging a photoconductor drum, using a charger, with a voltage which is the sum of a direct current (DC) voltage and an alternating current (AC) voltage.
2) Description of the Related Art
Two types of charging, contact and non-contact charging, are used in conventional image forming apparatuses. In the contact charging, a charger is in contact with a surface of a body to-be-charged. The charger is charged with a desired electric potential and that electric potential is transferred (or to inject) to the body. On the other hand, in the non-contact charging, the charger does not contact the body. Corona discharging, which is used widely in charging units, is a typical example of the non-contact charging.
The contact charging has following advantages: 1) the charging can be performed with low voltage; 2) less ozone is generated during charging so that an ozone filter is not required; 3) since the ozone filter is not required, the exhaust system becomes simple; 4) the charging unit does not require any maintenance; and 5) the charging unit is simple.
In the contact charging, one approach is to apply an AC-DC composite voltage, which is an AC voltage superimposed on a DC voltage, to the photoconductive drum. If the photoconductive drum is charged with the AC-DC composite voltage, a uniform potential can be applied on the surface of the photoconductive drum. This is because, a surface potential of the photoconductive drum converges to the DC component corresponding to a dark area potential Vd of the photoconductive drum, by the AC component.
However, impedance between a charging roller and the photoconductive drum fluctuates depending on the environment conditions. In other words, if the environment conditions change, the AC component causes defective charging or charge leak. To cope with this problem, the impedance is monitored, and the AC component is varied according to the variation in the impedance. This will be called as constant current control.
However, the constant current control has a drawback that, when the charging roller passes over pinholes on the photoconductive drum, electrical noise is produced and/or the impedance changes sharply. The electrical noise or the change in the impedance affects the current to be controlled and even drop the applied voltage. This results into defective charging and bad image quality.
Japanese Patent Application Laid-Open No. 5-11571 discloses an image forming apparatus that solves the above-mentioned problem. Precisely, the image forming apparatus detects current of a charging AC component, for example, when the power is turned on. Then, an AC output voltage is varied till the current reaches to a target value, the value of the output voltage is stored in a memory under a control of a CPU, and the AC component is controlled based on this output voltage for a prescribed time.
However, the conventional image forming apparatuses disclosed in the above-mentioned publication requires an AC current detection circuit to be installed in a high voltage power supply. As a result, following problems arise:
1) Extra bit(s) is required to be added to a control signal for the high voltage power supply; and
2) configuration becomes complicated and costly.
Precisely, regarding the first problem, since the detected current (analogue value of voltage into which the current is converted) is transmitted to the CPU (which is provided outside of the high voltage power supply), it is necessary to add one bit for current feed back to the control signal. For example, an image forming apparatus for four colors requires further four bits for the control signal.
Regarding the second problem, for example, the CPU needs to prepare a routine which varies the duty factor of pulse width modulation (hereinafter, xe2x80x9cPWMxe2x80x9d) signals to control the high voltage power supply such that an output current applied to the photoconductive drum indicates a desired value based on the AC current detected in a prescribed timing.
It is an object of the present invention to at least solve the problems in the conventional technology.
An image forming apparatus according to one aspect of the present invention includes a photoconductive drum to form a latent image; a charger that makes a contact with the photoconductive drum and charges the photoconductive drum; a developing unit that develops the latent image on the photoconductive drum; an environment information detection unit that detects current environment information about current environmental conditions around the photoconductive drum; a first memory unit that stores impedance characteristics data; and an impedance calculating unit that calculates an impedance between the charger and the photoconductive drum based on the present environment information and the impedance characteristics data.
A charging unit according to another aspect of the present invention includes an environment information detection unit that detects current environment information about current environmental conditions around a photoconductive drum forming a latent image; a first memory unit that stores impedance characteristics data; and an impedance calculating unit that calculates an impedance between the charger and the photoconductive drum based on the present environment information and the impedance characteristics data.
A method according to still another aspect of the present is of controlling a voltage applied to a charger making a contact with a photoconductive drum and charging the photoconductive drum. The method includes detecting current environment information about current environmental conditions around the photoconductive drum; calculating an impedance between the charger and the photoconductive drum based on the present environment information and impedance characteristics data; and generating the voltage which is an alternating current voltage superimposed on a direct current voltage, based on the impedance calculated.
The impedance characteristics data is obtained in advance by experiments and represents a relationship between environment information about environmental conditions around the photoconductive drum and impedance between the charger and the photoconductive drum.
The other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.